The invention concerns a warp-knit fabric of noble metal-containing wires as well as a method for mechanized production of gas-permeable warp-knit lattices of noble metal-containing wires by means of a knitting loom.
Knit materials made from noble metal-containing wires and methods for the production thereof have been known for some time. For example, catalyst lattices that consist of platinum-rhodium wires with thicknesses of between 60 and 120 .mu.m are used to catalyze the combustion of ammonia to produce nitric acid. In the corresponding combustion facilities the catalyst lattices are used, for example, transverse to the flow direction of the ammonia gas that is to be catalytically converted. Since the yield achieved in catalysis generally depends on the size of the free surface area coated with the catalytically effective material, the catalyst lattices are characterized by the largest possible surface area. At the same time, however, it is desirable for the gas flow to be impeded as little as possible, in other words for the flow resistance to be as low as possible.
Usually these catalyst lattices are manufactured from the respective noble metal wires by weaving them into webs, and are then cut to the desired dimensions. With a woven structure, however, the intersecting metal wires form only a two-layer structure of stitches. It has been found that a woven material of this type presents to a gas flow a relatively high resistance in relation to the surface area that is offered. Furthermore, the weaving of such catalyst lattices is relatively time-consuming due to the need to load the looms, and must also proceed slowly due to the limited ductility of the noble metal wires as compared to textile fibers.
EP-A1 03 64 153, to which U.S. Pat. No. 5,188,813 corresponds, describes a catalyst lattice and a method that are said to eliminate these drawbacks. In the method proposed therein for producing lattices of noble metal alloys, in particular for catalytic ammonia combustion and noble metal recovery, the noble metal alloy wires are weft-knit, together with an auxiliary thread made of natural fibers or a plastic, on a weft knitting loom. The looping which occurs during weft knitting produces a three-layer stitch structure that forms, for the same surface weight, a larger free surface area than with weaving. With this method, however, it is not possible to weft-knit pure noble metal alloy wires without auxiliary threads, since the wires break during the weft knitting process. Inclusion of the auxiliary thread has the disadvantage, however, that it must be removed before the lattices are used.
A similar method for producing gas-permeable lattices from noble metals by weft knitting wires made of platinum or palladium alloys on flat weft knitting looms is proposed in DE-C1 42 06 199. The applicability of this method is also limited, however, by the strength and ductility of the noble metal-containing wires, so that it is only suitable for weft knitting wires made of certain platinum-rhodium, platinum-palladium-rhodium, palladium-nickel, palladium-copper, and palladium-nickel-copper alloys with diameters between 50 and 120 .mu.m and tensile strengths between 900 and 1050 N/mm.sup.2.
In general, the known weft knitting methods using flat weft knitting looms are capable of producing only relatively coarse-mesh knit structures, which therefore have a low specific surface area in terms of the volume of the knit material and are mechanically labile. Although finer-mesh metal lattices can be produced by circular knitting, their web widths are limited to less than 3 meters by the circular knitting looms currently available.
A warp-knit fabric of this type and a method for producing it are described in WO 92/02301. This discloses catalyst and recovery lattices made of noble metal-containing wires, formed using a warp knitting loom with an open tricot weave.
In warp knitting as in weft knitting, the wires are looped to form a stitch structure with at least three layers, with which a greater surface area than with weaving can be obtained for the same surface weight. With the known warp knitting methods, however, only relatively loose lattices with little rigidity can be produced. In particular, the warp-knit structure formed with an open tricot weave is characterized by relatively low dimensional stability and easy displaceability, and has a relatively low specific surface area in terms of the volume occupied by the warp-knit fabric. To strengthen the open tricot weave, WO 92/02301 proposes relatively complex combination stitches, which can be produced for example with Raschel or Jacquard warp knitting techniques. Because of the low ductility and tensile and bending strength of the noble metal-containing wires as compared with textile fibers, however, knit weaves that place very high strength demands on the wires can be produced only to a limited extent or only at slow knitting speeds. In a preferred embodiment of this prior art, the noble metal-containing wires are therefore warp-knit together with a copper auxiliary thread or with synthetic fibers, or in an alternative procedure are coated with lubricant during knitting. This is designed to protect the wire against possible mechanical damage.